This application claims priority to Japanese Patent Application Number JP2002-076081 dated Mar. 19, 2002, which is incorporated herein by reference.
The present invention relates to a Solid State Image Pickup Device and a method of producing a solid state image pickup device for use as various kinds of image sensors and camera modules.
In recent years, the demand for video cameras and electronic cameras has increased dramatically and these cameras use a CCD type or amplification type solid state image pickup device.
Among these, the amplification type solid state image pickup device (CMOS image sensor) includes on a single semiconductor chip an image pickup pixel portion comprising a plurality of pixels arranged in a two-dimensional form, and a peripheral circuit portion disposed on the outside of the image pickup pixel portion. In each pixel of the image pickup pixel portion, floating diffusion (FD) portion as well as various MOS transistors including a transfer transistor and an amplification transistor are typically provided. In this case, light incident on each pixel is subjected to photo-electric conversion by a photodiode to generate a signal charge, the signal charge is transferred to the FD portion by the transfer transistor, the variation of potential at the FD portion is detected by the amplification transistor, and the detected variation is converted into an electric signal and amplified, whereby signals from each pixel are output through signal wires to the peripheral circuit portion.
In addition, the peripheral circuit portion is provided with a signal processing circuit for applying a predetermined signal processing, for example, CDS (correlative double sampling), gain control, A/D conversion, etc. to the pixel signals from the image pickup pixel portion, and a driving control circuit for controlling the output of the pixel signals by driving each pixel in the image pickup pixel portion, for example, vertical and horizontal scanners, a timing generator (TG), etc.
FIG. 11 is a sectional view showing a device structure in a related-art CMOS image sensor, and shows the structure of one pixel 10 in the image pickup pixel portion and one MOS transistor 20 provided in the peripheral circuit portion.
The pixel 10 in the image pickup pixel portion includes a P type well region 11 on an N type silicon substrate 1, and a photodiode 12 and an FD portion 13 are provided there. A polysilicon transfer electrode 14 for transfer gate for transferring a signal charge from the photodiode 12 to the FD portion 13 is provided in an upper insulating layer 2 of the N type silicon substrate 1, metallic wirings 15 and 16 formed of aluminum or the like are provided on the upper side of the polysilicon transfer electrode 14, and, further, a light-shielding film 17 having a light receiving opening portion for the photodiode 12 is provided on the upper side of the metallic wirings 15 and 16.
In addition, a passivation film 3 comprised of a silicon nitride film or the like is provided on the upper insulating layer 2, and an on-chip color filter 28 and an on-chip micro-lens 19 are provided on the upper side of the passivation film 3.
On the other hand, the MOS transistor 20 in the peripheral circuit portion is provided with a P type well region 21 on the N type silicon substrate 1, and a source region 22 and a drain region 23 are provided there. The upper insulating layer 2 of the N type silicon substrate 1 is provided with a polysilicon gate electrode 24 of the MOS transistor 20, metallic wirings 25, 26 and 27 formed of aluminum or the like are provided on the upper side of the polysilicon gate electrode 24, and, further, a metallic wiring 28 formed of aluminum or the like is provided also in the passivation film 3 on the upper side of the metallic wirings.
In the solid state image pickup device as described above, each pixel is so constructed that in order to enhance the numerical aperture of the photodiode 12 (the ratio of the incident light on the photodiode 12 to the incident light on the pixel), the incident light is condensed on the photodiode 12 through the portion between the wirings by a micro-lens 19.
In this case, however, a part of the light condensed by the micro-lens 19 is repelled by the wirings 15 and 16. This causes the following undesired problems.
(1) Sensitivity is lowered as much as the amount of the light repelled by the wirings.
(2) A part of the light repelled by the wirings enters into the photodiode of the adjacent pixel, resulting in mixture of colors.
(3) Since the layout of the wirings is restricted, characteristics are lowered by the limitations such that the wirings cannot be located on the upper side of the photodiode, or thick wirings cannot be utilized.
(4) Miniaturization is difficult to achieve for the same reason as (3) above.
(5) Since skew incidence of light occurs and the proportion of the light repelled is higher at the pixels in a peripheral area, dark shading occurs more heavily in the peripheral area.
(6) When it is intended to produce the CMOS image sensor by an advanced CMOS process in which the number of the wiring layers is increased further, the distance from the micro-lens to the photodiode is enlarged, and the above difficulties are further increased.
(7) Due to (6) above, the typical advanced CMOS processing techniques cannot be used, correction of the layout of the circuits registered in the library is needed, or the number of the wiring layers is limited and therefore the area is enlarged, so that the cost is raised. Besides, the pixel area per pixel is also increased.
In addition, when long-wavelength light such as red light is subjected to photo-electric conversion in the P type well region 11 deeper than the photodiode 12, the electrons generated diffuse into the P type well region 11, resulting in that the electrons enter into the photodiode 12 located at another position to cause mixing of colors. When the electrons enter into a pixel light-shielded for detection of black, the black level is detected erroneously.
Besides, while there is a process in which a silicide is used for the active region, the silicide hampers the incidence of light, so that a process of removing only the silicide on the photodiode 12 must be added.
Therefore, the number of steps is increased, and the process becomes complicated. In addition, defects in the photodiode arise from the steps, also.
Furthermore, such functions as a camera signal processing circuit and a DSP which have hitherto been composed of other chips are mounted on the peripheral circuit portion of the CMOS image sensor, as described above. As to these, since the process generation is advanced in the manner of 0.4 xcexcmxe2x86x920.25 xcexcmxe2x86x920.18 xcexcmxe2x86x920.13 xcexcm, the CMOS image sensor itself must be made to correspond to these new processes; if it is not fulfilled, the merits of miniaturization cannot be offered and the abundant library and knowledge of CMOS circuits cannot be utilized.
However, the number of layers in the wiring structure increases as the process generation advances. For example, while three wiring layers are used in the 0.4 xcexcm process, eight wiring layers are used in the 0.13 xcexcm process. In addition, the thickness of the wiring layer is also increased, and the distance from the micro-lens to the light receiving surface of the photodiode is increased by a factor of 3 to 5.
Therefore, in the related-art method in which the light is passed to the light-receiving surface through the wiring layers, it has come to be impossible to efficiently condense the light onto the light-receiving surface of the pixel, and the problems of (1) to (7) above have come to be conspicuous.
Meanwhile, recently, the so-called back-illuminated type solid state image pickup device in which the light-receiving surface of the photodiode is provided on the back side of a semiconductor chip has been proposed as a solid state image pickup device other than the above-mentioned CMOS image sensor.
This device is constructed as a frame transfer type CCD image pickup device, in which a silicon substrate is made to be a thin film, transfer electrodes and the like are provided on the face side of the thin film, and the light-receiving surface of the photodiode is disposed on the back side.
Then, the light received by the light-receiving surface is subjected to photo-electric conversion by the photodiode in the silicon substrate, and signal charges are trapped by a depletion layer extending from the substrate face side, are accumulated in a potential well (P+ type well region) on the face side, and transferred and outputted.
FIG. 12 is a sectional view showing the device structure of a photodiode portion in such a back-illuminated type solid state image pickup device.
The solid state image pickup device has a structure in which an epitaxially grown N type well region 31 is provided on a thin film type Pxe2x88x92 type silicon substrate 30, and a P+ type well region 33 is provided on the upper side of the N type well region 31 through a depletion layer 32 therebetween to compose a photodiode.
Moreover, an oxide film 34 and an aluminum light-shielding film 35 are provided on the P+ type well region 33.
In addition, the side of the Pxe2x88x92 type silicon substrate 30 is the back side, i.e., the light-illuminated side, and the side of the oxide film 34 and the aluminum light-shielding film 35 is the face side, where wirings for transfer electrodes, for example, and the like are disposed.
However, the image pickup device with such a structure has the problem the sensitivity for blue color for which absorptivity is high is lowered. In addition, since light is incident on the back side and is subjected to photo-electric conversion at a shallow position, the signal charges generated diffuse and, in a proportion, would enter into the photodiodes in the surroundings.
On the other hand, in the case of the CCD type image pickup device, it is needless to enlarge the height of the wiring layer since system-on-chip is not adopted, and the light-shielding film can be dropped into the surroundings of the photodiode since a process peculiar to CCD is adopted, so that condensation of light by an on-chip lens is easy to achieve. Therefore, in the case of the CCD type image pickup device, the above-mentioned problems (1) to (7) encountered in the case of the CMOS image sensor are not generated.
From the above circumstances, the back-illuminated type CCD type image pickup device has almost not been put to practical use, and such a back-illuminated type CCD image pickup device in which color filters and micro-lenses are in an on-chip form are not typically utilized.
In contrast, in the case of the CMOS image sensor, a process obtained by slight correction to a standard CMOS process is used. Therefore, the CMOS image sensor has the merit which is not possessed by the CCD type image pickup device, such that, by adopting the above-mentioned back-illuminated type, a newest process can always be used without being influenced by the wiring step.
In addition, the structure in which a number of layers of metallic wirings extend in crossing directions is absent in the case of the CCD type image pickup device. Therefore, different from the case of the CCD type image pickup device, the above-mentioned problems (1) to (7) are conspicuous particularly in the case of the CMOS image sensor. From this point of view, also, adoption of the back-illuminated type for the CMOS image sensor is advantageous.
However, on one hand, at the time of forming color filters and on-chip micro-lenses on a wafer of an ordinary CMOS image sensor, registration (positioning) of a stepper is conducted by use of the metallic wiring layer formed of aluminum or the like. On the other hand, at the time of producing the back-illuminated type CMOS image sensor, after the wiring step for the wafer is completed, the wafer is inverted face side back, the side opposite to the side where the wiring is provided is polished, then formation of a silicon oxide film (SiO2), formation of a light-shielding film and formation of a passivation film are conducted, and thereafter formation of back-side color filters and back-side micro-lenses is conducted.
Therefore, in the case of producing the back-illuminated type CMOS image sensor, there is the problem that the registration mark formed at the time of producing the aluminum wiring layer cannot be used as it is as in a related art.
Accordingly, it is an object of the present invention is to provide a method of producing a solid state image pickup device in which various kinds of registrations at the time of producing the so-called back-illuminated type amplification type solid state image pickup device (CMOS image sensor) can be provided easily and appropriately, and production efficiency and device accuracy can be improved.
In order to attain the above object, according to the present invention, there is provided a method of producing a solid state image pickup device comprising a semiconductor substrate provided with an image pickup pixel portion in which a plurality of pixels each comprising a photo-electric conversion device and a field effect transistor are arranged in a two-dimensional array. A peripheral circuit portion comprising a driving circuit for driving the image pickup pixel portion and a signal processing circuit for processing a pixel signal outputted from the image pickup pixel portion, and a wiring layer, are also provided on a first side of the semiconductor substrate. The peripheral drive circuit is used for driving the field effect transistors in the image pickup pixel portion. A light-receiving surface of the photo-electric conversion device is formed on a second side of the semiconductor substrate, wherein a registration mark is formed by use of an active region or a gate layer for the field effect transistors arranged on the first side of the semiconductor substrate, and registration of each device on the second side in the subsequent step is achieved by use of the registration mark.
In accordance with the method of producing a solid state image pickup device of the present invention, the registration mark is formed by using the active region or the gate layer for the field effect transistors arranged on the wiring side (first side) opposite to the illuminated side of the semiconductor substrate, whereby the registration for each device on the second side which has been difficult to achieve by use of a metallic wiring layer as in the related art can be achieved by detecting the registration mark formed on the first side of the semiconductor substrate through the thin film semiconductor substrate.
Therefore, the desired positioning and registration for each device can be conducted easily and appropriately, without applying any special registration means to the second side of the semiconductor substrate, and production efficiency and device accuracy can be significantly improved.
Also, there is provided a solid state image pickup device comprising a substrate, a first alignment mark formed on a first surface of said substrate , a micro-lens formed on a second surface of said substrate, wherein said alignment mark is formed only on a surface portion of said first surface, and wherein a position of said micro-lens has a constant relationship with that of said alignment mark.
Further, there is provided a solid state image pickup device comprising a photo-electric conversion region, a MOS transistor formed on a first surface of a substrate, wherein said photo-electric conversion region is comprised of a first impurity region of a first conductivity type formed on a second surface of said substrate, a second impurity region of a second conductivity type formed on said first impurity region and a third impurity region of said first conductivity type formed on said second impurity region, and wherein said MOS transistor is comprised of a first well of said first conductivity type and a source and a drain regions of said second conductivity type formed in said first well.